I highly believe the person behind these coins had indeed lost access and found it after 14 years. You can't have this level of self-control to not spend any coin.
Since we're speculating now: I don't think the keys were lost. As I wrote above (4 years ago), it was worth $30k when it was received. That doesn't sounds like something to lose access to. So my guess is the owner had multiple more addresses, and may have sold some other coins over the years. Just not this input.A billion bucks
That's quite literally a truckload of money.